1. Field of the Invention
The present invention relates to a nitride-based semiconductor light-emitting device, and more particularly, it relates to a nitride-based semiconductor light-emitting device comprising nitride-based semiconductor layers.
2. Description of the Background Art
A nitride-based semiconductor light-emitting device such as a nitride-based semiconductor light-emitting diode device is actively developed at present. In order to employ a nitride-based semiconductor light-emitting diode device as the light source for a lighting apparatus, improvement of the light output characteristic of the nitride-based semiconductor light-emitting diode device and increase of an applied current have recently been developed in particular. In order to form such a nitride-based semiconductor light-emitting diode device, nitride-based semiconductor layers are grown on a sapphire substrate requiring a lower cost than a high-priced nitride-based semiconductor substrate of GaN or the like.
FIG. 48 is a sectional view showing the structure of a conventional nitride-based semiconductor light-emitting diode device. Referring to FIG. 48, a buffer layer 152, an underlayer 153, an n-type contact layer 154, an n-type cladding layer 155 and an active layer 156 are successively formed on an insulating sapphire substrate 151 in the conventional nitride-based semiconductor light-emitting diode device. A cap layer 157, a p-type cladding layer 158 and a p-type contact layer 159 are successively formed on the active layer 156. The n-type contact layer 154, the n-type cladding layer 155, the active layer 156, the cap layer 157, the p-type cladding layer 158 and the p-type contact layer 159 constitute a nitride-based semiconductor element layer 150.
A prescribed region of the nitride-based semiconductor element layer 150 is removed between the upper surface of the p-type contact layer 159 and a portion of an intermediate depth of the n-type contact layer 154, for partially exposing the n-type contact layer 154. A p-side electrode 160 is formed on the p-type contact layer 159, while an n-side electrode 161 is formed on a prescribed region of the exposed surface of the n-type contact layer 154.
In the conventional nitride-based semiconductor light-emitting diode device shown in FIG. 48, as hereinabove described, the p-side electrode 160 and the n-side electrode 161 are taken out from the surface of the nitride-based semiconductor element layer 150 opposite to the sapphire substrate 151. In order to increase the emission area for improving the light output characteristic, therefore, light must be emitted from the side of the sapphire substrate 151 formed with neither the p-side electrode 160 nor the n-side electrode 161. Therefore, flip chip bonding is employed for mounting the conventional nitride-based semiconductor light-emitting diode device shown in FIG. 48 on a base (not shown) from the side of the p-side electrode 160 and the n-side electrode 161.
However, the conventional nitride-based semiconductor light-emitting diode device shown in FIG. 48 has a step between the p-side electrode 160 formed on the p-type contact layer 159 and the n-side electrode 161 formed on the exposed surface of the n-type contact layer 154. In order to assemble the conventional nitride-based semiconductor light-emitting diode device shown in FIG. 48 by flip chip bonding, therefore, the base must be provided with a step portion corresponding to the step between the p-side electrode 160 and the n-side electrode 161, and precise position control is required for aligning the positions of the step portion and the p- and n-side electrodes 160 and 161 with each other. Therefore, the manufacturing yield is disadvantageously reduced.
In relation to this, a nitride-based semiconductor light-emitting diode device formed by growing a nitride-based semiconductor element layer on a sapphire substrate and thereafter bonding a cleavable conductive substrate of GaAs or the like to the nitride-based semiconductor element layer and removing the sapphire substrate is proposed in general, as disclosed in Japanese Patent Laying-Open No. 9-8403 (1997), for example. In the aforementioned nitride-based semiconductor light-emitting diode device disclosed in Japanese Patent Laying-Open No. 9-8403, a p-side electrode and an n-side electrode can be oppositely arranged through the nitride-based semiconductor element layer, due to the employment of the conductive substrate. Therefore, the nitride-based semiconductor light-emitting diode device may not be assembled by flip chip bonding requiring precise position control dissimilarly to the case of employing the insulating sapphire substrate, whereby the manufacturing yield can be improved.
In the aforementioned nitride-based semiconductor light-emitting diode device disclosed in Japanese Patent Laying-Open No. 9-8403, the nitride-based semiconductor element layer bonded to the conductive substrate is formed by a p-type nitride-based semiconductor layer, an active layer and an n-type nitride-based semiconductor layer successively from the side closer to the conductive substrate, for emitting light from the surface of the n-type nitride-based semiconductor layer. However, impurity levels are formed in the n-type nitride-based semiconductor layer doped with an n-type impurity, to result in light absorption. Consequently, the n-type nitride-based semiconductor layer easily absorbs light totally reflected on the surface thereof, to disadvantageously reduce light extraction efficiency.